The present invention relates to a coating composition capable of forming an alkali-soluble lubricating film suitable for shape working, to lubricating surface treated metal articles coated with the coating composition, and to a process for production of metal articles that are coated with the coating composition and shape worked.
Working processes such as press molding of steel sheets have conventionally been carried out while applying lubricating oils, and the like, for the purpose of preventing scratches to the steel sheet surface and molding die surface that occur due to lack of lubrication. This has not only complicated the production process and impaired the working environment, but has necessitated lubricant removal using solvents such as fluon, trichloroethane, dichloromethane and the like in the degreasing step after press molding; such solvents are undesirable from the standpoint of worker health and the environment.
Much study has therefore been prompted by the desire to provide lubricating, surface-treated metal articles capable of being press molded into desired shapes without using such solvents. There have been developed non-strippable lubricant films exhibiting workability, corrosion resistance, solvent resistance, etc. as lubricant films that remain even after press molding, and strippable lubricant films wherein the lubricant films are dissolved and degreased during the alkali degreasing step after press molding. Alkali-strippable lubricating films are used to bring out attractive metal surfaces and for purposes that require weldability after working.
In Japanese Unexamined Patent Publication HEI No. 8-156177, No. 8-252887 and No. 10-114014 there have been proposed alkali-strippable lubricating films employing acrylic-based resins. However, alkali-strippable lubricating films employing acrylic-based resins often fail to offer adequate shapeability, as they undergo scratching under severe press molding conditions such as deep drawing or wipe working.
It is an object of the present invention to solve the problems mentioned above by providing a coating composition with excellent press moldability and scratch resistance, which can form alkali-soluble lubricating films suitable for shape working, as well as lubricating surface treated metal articles coated with the coating composition.
The present inventors have conducted much diligent research aimed at solving the aforementioned problems and obtaining a coating composition with excellent press moldability and scratch resistance, which can form alkali-soluble lubricating films suitable for shape working, as well as lubricating surface treated metal articles. As a result they have completed the present invention upon finding that this object can be achieved by a coating composition comprising an aqueous polyurethane composition and a lubriction functionality-providing agent.
Specifically, the coating composition capable of forming an alkali-soluble lubricating film according to the invention (1) is characterized in that it contains as main components an aqueous polyurethane composition (A) and a lubricating functionality-providing agent (B) at 1-30 wt % with respect to the solid content of the aqueous polyurethane composition, and in that the coating composition is film-formable and its formed films are alkali-soluble. This coating composition (1) that comprises an aqueous polyurethane composition (A) and a lubricating functionality-providing agent (B) exhibits sufficient lubricating properties and ground layer protection even under severe shape working conditions such as press working, deep drawing, wipe working and roll forming.
The coating composition capable of forming an alkali-soluble lubricating film according to the invention (2) is characterized in that it contains, in addition to the aqueous polyurethane composition (A) and the lubricating functionality-providing agent (B), also silica particles (C) at 1-30 wt % with respect to the solid content of the aqueous polyurethane composition.
The coating composition capable of forming an alkali-soluble lubricating film according to the invention (3) is a coating composition according to (1) or (2) above characterized in that the aqueous polyurethane composition (A) comprises a polyester polyol. This aqueous polyurethane composition (A) that comprises a polyester polyol can easily provide the alkali solubility required for the invention.
The coating composition capable of forming an alkali-soluble lubricating film according to the invention (4) is a coating composition according to (1) or (2) above characterized in that the aqueous polyurethane composition (A) contains a carboxyl group or sulfonic acid group, or a salt thereof, as a hydrophilic group. The aqueous polyurethane composition (A) wherein the hydrophilic group is a carboxyl group or sulfonic acid group or a salt thereof can exhibit excellent adhesion with metal sheet surfaces, and provide adequate shapeability under severe press molding conditions such as deep drawing and wipe working.
The coating composition capable of forming an alkali-soluble lubricating film according to the invention (5) is a coating composition according to (1) or (2) above characterized in that the hydrophilic group of the aqueous polyurethane composition (A) is a Na salt or K salt of a carboxyl group or sulfonic acid group. The carboxyl group or sulfonic acid group as the hydrophilic group of the aqueous polyurethane composition (A) is neutralized by the Na or K to achieve a satisfactory stripping property with alkali degreasing agents.
The coating composition capable of forming an alkali-soluble lubricating film according to the invention (6) is a coating composition according to (1) or (2) above characterized in that the carboxyl group or sulfonic acid group as the hydrophilic group of the aqueous polyurethane composition (A), or its salt, is included to an acid value in the range of 25-180. The aqueous polyurethane composition (A) wherein the carboxyl group or sulfonic acid group as the hydrophilic group, or its salt, is included to an acid value in the range of 25-180, provides a satisfactory stripping property with alkali degreasing agents even at room temperature.
The coating composition capable of forming an alkali-soluble lubricating film according to the invention (7) is a coating composition according to (3) above characterized in that the polyester polyol in the aqueous polyurethane composition (A) is prepared by reacting ethylene glycol with an aliphatic dibasic acid or its dialkyl ester, or a mixture thereof. The aqueous polyurethane composition (A) comprising a polyester polyol composed of ethylene glycol and a dicarboxylic acid can easily provide the alkali solubility required for the invention even at lower temperatures such as room temperature.
The coating composition capable of forming an alkali-soluble lubricating film according to the invention (8) is a coating composition according to (1) or a coating composition according to (2) above characterized in that the hydrophilic group of the aqueous polyurethane composition (A) is a carboxyl group or sulfonic acid group, or a salt thereof. The aqueous polyurethane composition (A) wherein the hydrophilic group is a carboxyl group or sulfonic acid group or salt thereof can exhibit excellent adhesion with metal sheet surfaces, and provide adequate shapeability under severe press molding conditions such as deep drawing and wipe working.
The coating composition capable of forming an alkali-soluble lubricating film according to the invention (6) is a coating composition according to (1) to (5) above, characterized in that the lubricating functionality-providing agent (B) comprises one or more from among polyolefin-based waxes, fluorine-based waxes, paraffin-based waxes and stearic acid-based waxes. The lubricating surface treated metal articles of the invention are steel sheets with excellent press moldability and scratch resistance, characterized by being coated with any of the aforementioned coating compositions to a dry film thickness of 0.5-5 xcexcm.
According to the invention there are provided surface treated metal articles obtained by coating metal surfaces with the aforementioned coating compositions capable of forming alkali-soluble lubricating films, and a process for production of metal articles involving shape working of such surface treated metal articles.
Specifically, a lubricating surface treated metal article according to the invention is characterized in that a lubricating film comprising a polyurethane resin (Axe2x80x2) and a lubricating functionality-providing agent (B) at 1-30 wt % with respect to the polyurethane resin, is formed on the surface of a metal article, wherein the lubricating film is alkali soluble and the film thickness is 0.5-5 xcexcm.
The process for production of metal articles according to the invention is characterized by comprising
i) a step of coating the surface of a metal article with a coating composition containing an aqueous polyurethane composition (A) and a lubricating functionality-providing agent (B) at 1-30 wt % with respect to the solid content of the aqueous polyurethane composition (A), the aqueous polyurethane composition being film-formable and the formed film being alkali soluble, to form an alkali-soluble lubricating film,
ii) a step of shape working the metal article surface treated with the alkali-soluble lubricating film, and
iii) a step of treating the shape worked metal article with an alkali to remove the alkali-soluble lubricating film.